1. Field of the Invention
The present invention relates to an optical transmission system for amplifying and relaying a plurality of optical wavelength signals in a wide band; and an optical amplifying apparatus, an optical amplifying unit, and an optical coupler which are suitably used in this optical transmission system.
2. Related Background Art
Wavelength division multiplexing (WDM) transmission systems, which transmit a plurality of optical wavelength signals, have been known as a system which can carry out high-speed, large-capacity optical communications. Since the transmission loss of optical silica-based fibers used as optical transmission lines is low in the vicinity of a wavelength of 1.55 xcexcm, and C-band optical amplifiers for amplifying optical signals in C band (1.55-xcexcm wavelength band, 1530 nm to 1562 nm in general) have been in practical use, such a WDM transmission system uses a plurality of optical wavelength signals in C band.
Recently, however, since further higher speed and larger capacity have been demanded, while L-band optical amplifiers for amplifying optical signals in L band (1.58-xcexcm wavelength band, 1574 nm to 1605 nm in general) have been under development, the use of L-band optical signals in addition to C-band optical signals for carrying out WDM transmissions has been under consideration. Examples of literatures disclosing the same include M. X. Ma et al., xe2x80x9c765 Gb/s over 2,000 km Transmission Using C- and L-band Erbium Doped Fiber Amplifiers,xe2x80x9d OFC""99, Postdeadline papers, PD16 (1999), and the like.
It is necessary for optical amplifying apparatus in such an optical transmission system to optically amplify optical signals in a band including both of C band and L band. Therefore, it is a common practice to provide dedicated optical amplifiers for C band and L band, respectively. Namely, the optical amplifying apparatus if configured such that an inputted multiplexed optical signal is separated by an optical branching section into a C band multiplexed optical signal and an L band multiplexed optical signal, the C band multiplexed optical signal is optically amplified by the C-band optical amplifier, the L band multiplexed optical signal is optically amplified by the L-band optical amplifier, thus optically amplified multiplexed optical signals are combined by an optical combiner, and the resulting combined optical signal is outputted.
As the above-mentioned C-band optical amplifier and L-band optical amplifier, Er-doped fiber amplifiers (EDFA) are in wide use. The inventor has found that, when such an EDFA is used for constructing the above-mentioned WDM transmission system, there are problems as follows.
Namely, backward ASE (Amplified Spontaneous Emission) light has a peak not in L band but in C band, and the interband isolation in the optical branching section is not perfect. Therefore, due to Rayleigh scattering in the optical fiber line connected to the upstream stage of the optical branching section, the backward ASE light generated in the L-band optical amplifier is inputted to the C-band optical amplifier as return light by way of the optical branching section, so as to become noise. Therefore, for securing a favorable optical SN ratio, it is inevitable to reduce the number of stages of amplification and relay, which forms a bottleneck in long-distance transmissions.
For overcoming the above-mentioned problems according to the above-mentioned finding of the inventor, it is an object of the present invention to provide an optical transmission system, suitable for long-distance transmissions, which can increase the number of stages of amplification and relay while securing a favorable optical SN ratio; and an optical amplifying apparatus, an optical amplifying unit, and an optical coupler used in this optical transmission system.
Namely, the optical transmission system in accordance with the present invention is an optical transmission system for transmitting a multiplexed optical signal belonging to two wavelength bands to first and second transmission lines which are connected to each other by way of an amplifying repeater station, the amplifying repeater station comprising: (1) an optical branching section, connected to the first transmission line, for separating the multiplexed optical signal into first and second wavelength bands different from each other and outputting thus separated multiplexed optical signals individually, while transmitting therethrough return light from a branch thereof to the first transmission line; (2) two optical amplifiers, each having an amplification optical waveguide doped with a predetermined fluorescent material having a fluorescent spectrum peak in the first wavelength band, connected to two outputs of the optical branching section, respectively, for amplifying the each of the multiplexed optical signals; (3) an optical combiner, connected to respective outputs of the optical amplifiers, for combining the amplified multiplexed optical signals and outputting thus combined optical signal to the second transmission line; and (4) an optical filter, connected between the optical amplifier for amplifying the multiplexed optical signal in the second wavelength band and the optical branching section, for blocking light in the vicinity of the fluorescent spectrum peak wavelength and transmitting therethrough light in the second wavelength band.
The optical amplifying apparatus in accordance with the present invention is suitably used in such an optical transmission system, and comprises: (1) an optical branching section for separating the multiplexed optical signal into first and second wavelength bands different from each other and outputting thus separated multiplexed optical signals individually; (2) two optical amplifiers, each having an amplification optical waveguide doped with a predetermined fluorescent material having a fluorescent spectrum peak in the first wavelength band, connected to two outputs of the optical branching section, respectively, for amplifying the each of the separated multiplexed optical signals; (3) an optical combiner, connected to respective outputs of the optical amplifiers, for combining the amplified multiplexed optical signals and outputting thus combined optical signal; and (4) an optical filter, connected between the optical amplifier for amplifying the multiplexed optical signal in the second wavelength band and the optical branching section, for blocking light in the vicinity of the fluorescent spectrum peak wavelength and transmitting therethrough light in the second wavelength band.
The optical amplifying unit in accordance with the present invention is an optical amplifying unit suitably used in the above-mentioned optical transmission system, and comprises: (1) an optical amplifier, having an amplification optical waveguide doped with a predetermined fluorescent material, for amplifying a multiplexed optical signal in a wavelength band different from a wavelength band including a fluorescent spectrum peak of the fluorescent material; and (2) an optical filter, connected to an input end of the optical amplifier, for blocking light in the vicinity of the fluorescent spectrum peak wavelength of the fluorescent material and transmitting therethrough light in the wavelength band to be amplified by the optical amplifier.
The optical coupler in accordance with the present invention is an optical coupler suitably used in the above-mentioned optical transmission system, and comprises: (1) an optical branching section for separating the multiplexed optical signal into first and second wavelength bands different from each other and outputting thus separated multiplexed optical signals individually; and (2) an optical filter, connected to an output end of the optical branching section for the multiplexed optical signal in the second wavelength band, for guiding the optical signal to an optical amplifier having an optical amplification waveguide doped with a fluorescent material having a fluorescent spectrum peak in the first wavelength band, the optical filter blocking light in the vicinity of the fluorescent spectrum peak wavelength and transmitting therethrough light in the second wavelength band.
In the optical transmission system and optical amplifying apparatus in accordance with the present invention, a multiplexed optical signal inputted to the amplifying repeater station or optical amplifying apparatus is separated by the optical branching section into two bands in terms of wavelength bands, and the optical signals thus separated are outputted individually. The multiplexed optical signals in their respective wavelength bands are amplified by their corresponding different optical amplifiers, and then are combined by the optical combiner so as to be outputted.
Here, though the same fluorescent material is added to the respective optical waveguides of the two optical amplifiers, the population inversion of one optical waveguide is made different from that of the other optical waveguide, so that their respective gain bands shift from each other, whereby the wavelength bands to be amplified are changed. In this case, backward ASE light in a band in the vicinity of the fluorescent spectrum peak wavelength can occur in any of the optical amplifiers. In accordance with the present invention, however, since the backward ASE light generated upon optical amplification of the second wavelength band is blocked by the optical filter, the backward ASE light is prevented from being inputted to the optical amplifier for the first wavelength band, which is a band including this backward ASE light, even when the interband isolation is imperfect in the optical branching section. Therefore, the deterioration of noise characteristics is restrained in the optical amplifier for the first wavelength band.
As a result, the optically amplifiable optical signal wavelength band widens in each optical amplifying apparatus, and the noise characteristic of each optical amplifying apparatus becomes better. Therefore, even when the number of stages of amplification and relay is greater than that in conventional cases, a favorable optical SN ratio can be secured, and large-capacity, long-distance communications can be carried out.
Preferably, the optical branching section, the optical filter, and the optical amplifier for the second wavelength band are connected in series in this order. It is due to the fact that, if an optical isolator, for example, is inserted between the optical amplifier for the second wavelength band and the optical branching section, then the SN ratio after the amplification will deteriorate due to the resulting insertion loss, whereby the number of stages of amplification will be limited.
Preferably, the optical filter is an optical fiber grating in which a grating is formed in a core region of an optical fiber. In this case, the insertion loss with respect to the light in the second wavelength band to be transmitted through the optical filter is small, and fusion loss is small when the optical filter is connected to an optical fiber by fusion. Further, it will be preferable if the optical fiber formed with the optical fiber grating and optical fibers connected to both ends thereof are of the same kind, since the fusion loss becomes substantially zero as a consequence.
Preferably, the fluorescent material added to each of the amplification optical waveguides is Er element or Tm element. It is due to the fact that efficient optical amplification is carried out in a wavelength band where the transmission loss of a optical silica-based fiber used as the optical transmission line is small.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings. They are given by way of illustration only, and thus should not be considered limitative of the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it is clear that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, and various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.